Before you start to pull, you need to know how to hang. I recommend starting here with How to do your first Pull-Up: Part 1 and working through the progressions.
Once you are comfortable hanging from the bar, and you have adequate scapular strength, mobility, and control, you can move on to the Pull-Up. If you have followed the hanging progressions, chances are very good you can already struggle your way up to the bar. It might not be perfect, but it is a start. The purpose of Part 1 was to develop the proper starting position and kinesthetic awareness (i.e. body awareness) required to build a strong pull. In Part 2 we are focusing on the actual pull. I still recommend your continue your hanging training, in order to maintain your shoulder health.